1. Field of the Invention
The present invention relates generally to the field of comparators, and more particularly to the field of high speed comparators for use in low level logic circuits.
2. Prior Art
One important operation which must be performed in many electronic logic circuits is that of comparison of the amplitude of two voltages, with the resulting signal being suitable for use with logic circuitry. This can be done by simply using a differential amplifier and noting whether the output is positive or negative with respect to a predetermined zero level as an indication of which voltage is higher. Since output voltage swings of at least 200 millivolts are needed to operate low level logic circuits, relatively high gain amplifiers are required to compare low voltage differences of the order of 2 millivolts. When the differential voltage to be detected is relatively high and/or the speed with which the detection must be accomplished is slow, this approach may be satisfactory, but if the differential voltage is of the order of a few millivolts or less and detection must be accomplished in a few nanoseconds, it is difficult to provide the circuit bandwidth required for reliable operation. This is particularly true for applications requiring a large number of comparators, such as high speed parallel A/D converters where complexities must be minimized.
In general, techniques to improve switching time response result in reduced sensitivity. For example, it is common to provide positive feedback around an amplifier to cause it to switch rapidly from one state to another, but in so doing, a dead zone is created with respect to the input voltage so that the input voltage resolution is impaired.
One well known way of assessing the relative performance of comparators is by comparison of their effective gain-bandwidth products. The higher the gain-bandwidth product, the better is the performance. Prior art comparators of the types described above have gain-bandwidth products of the order of 300 to 400 megahertz whereas the effective gain-bandwidth product of an exemplary embodiment of the present invention is of the order of 30 gigahertz.
One previously known way of improving the response of a comparator having positive feedback as described above is to apply the positive feedback only after the differential voltage is applied using a strobe or clock pulse, so as to avoid the dead zone problem occasioned by the positive feedback. Such a comparator has a much improved gain-bandwidth product, perhaps of the order of 25 gigahertz, still below that of the present invention. Further, for equivalent complexity, the comparator of the present invention will provide an input impedance substantially higher (of the order of three to five times) than that of strobed feedback comparators known in the prior art.